Currently, most aluminum alloy heat exchangers, particularly heat exchangers for air conditioners of motor cars, are produced by combining and bonding molded fins and tubes with each other by a vacuum brazing procedure (VB method) or a non-corrosive flux-brazing procedure. Usually, the aluminum alloy for the heat exchangers is selected from aluminum-magnesium alloys (NB method). The brazing conditions are variable in response to the types of the aluminum alloys and the brazing methods. Usually, the brazing procedure is carried out at a high temperature of 550° C. or more and, thus, an oxide layer is usually formed on the surfaces of the fins and tubes. Also, in the case where the VB method is applied, magnesium (Mg) contained as a gettering agent in the alloy works to destroy the oxide layer and is converted to magnesium oxide, and the resultant magnesium oxide adheres to the surfaces of the aluminum alloy base body. Therefore, the surfaces of the base body is covered with the magnesium oxides of Al and Mg. Further, in the case where the NB method is applied, it causes the non-corrosive flux to remain in a large amount on the surfaces of the base body. As mentioned above, the surfaces of the heat exchanger are in various conditions different from those of the pure aluminum oxide layer.
The surfaces of the aluminum alloy heat exchangers having the above-mentioned surface conditions are mostly subjected to a chemical conversion treatment and then a hydrophilicity-enhancing treatment, to impart a corrosion resistance or to impart an enhanced hydrophilicity to the heat exchangers, for example, evaporators of air conditioners of motor cars, for the purpose of preventing scattering of water droplets. To form the chemical conversion coating in a sound condition, the various types of oxides or flux must be removed from the surfaces of the heat exchanger in the above mentioned conditions. For this purpose, usually, a chemical etching procedure is applied to the heat exchanger surfaces before the chemical conversion treatment procedure. The chemical conversion procedures are disclosed in, for example, Japanese Examined Patent Publication No. 6-12217 (Patent document 1) and Japanese Unexamined Patent Publication No. 9-14889 (Patent document 2).
The chemical etching procedures for the aluminum alloys include a method using an aqueous acid solution and a method using an aqueous alkaline solution. In the case where the covering layer formed on the base body surfaces is composed of natural oxides, generally the etching procedure for the covering layer can be carried out by using the aqueous alkaline solution at a high etching rate with a high process efficiency. However, on the surfaces of the heat exchanger assembled by the vacuum brazing procedure, a magnesium (Mg) oxide layer is further generated, and the magnesium oxide has a poor solubility in the aqueous alkaline solution. Therefore, in this case, the etching procedure using the aqueous alkaline solution exhibits a low etching rate. Also, due to the poor solubility of the magnesium oxide in the aqueous alkaline solution, the etching effect on the magnesium oxide layer becomes uneven, and usually, the resultant etched surface exhibits a serious roughness. This etching result mostly causes the chemical conversion coatings formed by the chemical conversion procedure applied to the etched surfaces to be uneven and, thus, the properties, for example, corrosion resistance, of the chemical conversion coatings may be insufficient. Further, when the hydrophilicity-enhancing procedure is applied on the chemical conversion coating surfaces having the serious roughness, the resultant hydrophilic coatings have an uneven thickness and, thus, after a lapse of time, the chemical conversion coating laying under the hydrophilic layer may be partly exposed to the outside to cause the hydrophilicity of the heat exchanger surfaces to be uneven. To prevent this disadvantage, the hydrophilic layer must be formed in a large amount which may cause an economical disadvantage to occur.
Under the above-mentioned circumstances, as a pretreatment for the aluminum alloy heat exchangers, a pre-treatment with an aqueous acid solution is proposed. For example, Japanese Unexamined Patent Publication 2001-158983 (Patent document 3) discloses an acid cleaning agent for a chemical conversion coating of a heating exchanger, an acid cleaning method of a heat exchanger, a method of treating a heat exchanger and a heat exchanger. In the acid cleaning method, an aluminum alloy heat exchanger is cleaned with an aqueous acid solution containing at least one member selected from Fe, Ni, Co, Mo and Ce metals and/or salts of the metals and nitric acid or sulfuric acid. This method is effective to enhance the corrosion resistance of the heat exchanger. However, to increase the rate of cleaning due to etching with the acid, the concentration of the etching agent and the etching temperature must be increased. Particularly, in the case where sulfuric acid is used, a problem of corroding the etching apparatus with the acid cannot be avoided. To prevent the corrosion, a large investment is necessary. Also, when nitric acid is used, waste water delivered from the etching procedure contains a large amount of nitrogen which is undesirable in view of environmental pollution.
Further, Japanese Unexamined Patent Publication No. 11-264088 (Patent document 4) discloses a pretreatment method of a surface treatment for an aluminum alloy article which does not relate to the heat exchangers. In this method, an aluminum alloy article containing silicon (Si) is cleaned with an acid liquid comprising nitric acid, ammonium fluoride and acetic acid. This method is carried out at about room temperature. The treating liquid contains nitric acid and acetic acid each in a high concentration and thus the method is not preferable in view of a pollution of process environment and of a difficulty in treatment of waste liquid delivered from the method procedure.
Accordingly, a surface treatment method comprising an etching step with an aqueous alkaline solution, in which step a rapid treatment can be carried out while problems in the investment for equipment, the environmental pollution of the method procedure and the treatment of the delivered waste liquid can be avoided, and capable of forming a uniform chemical conversion coating which does not cause the hydrophilic property of the surface to be uneven, is strongly desired.
Japanese Unexamined Patent Publication No. 5-51765 (Patent document 5) discloses a surface treating method for an aluminum material having a surface oxide layer with an aqueous solution of an cheleting agent and an organic amine compound, to thereby removing the surface oxide layer.
Japanese Unexamined Patent Publication No. 11-12796 (Patent document 6) discloses a method of producing a surface-treatment aluminum material for a two-piece can. In this method, an aluminum or aluminum alloy material is subjected to an electrolysis treatment in an aqueous electrolytical solution comprising at least one member selected from boric acid, borate salt, adipic acid, tartarate salts, citrate salts and malonate salts, to thereby form a non-porous anodic oxidation coating having a water content of 5% or less on the aluminum or aluminum alloy surface, and then to a lamination of an organic resin coating/on the non-porous anodic oxidation coating.                Patent document 1: Japanese Examined Patent Publication No. 6-12217, pages 1 to 2        Patent document 2: Japanese Unexamined Patent Publication No. 9-14889, page 2        Patent document 3: Japanese Unexamined Patent Publication No. 2001-158983, page 2        Patent document 4: Japanese Unexamined Patent Publication No. 11-264088, page 2        Patent document 5: Japanese Unexamined Patent Publication No. 5-51765        Patent document 6: Japanese Unexamined Patent Publication No. 11-12796        